When using laser plotters for high resolution pattern drawing, a problem arises when attempting to write on large areas high resolution pixels having a small size. In the electronics industry, the density of features in a given plot increases whilst there is no compensating decrease in the required image area. In the field of graphic arts, printing plates must accommodate ever increasing numbers of pages per single plate. In both cases, the requirements are even more severe when the written medium is flat, such as a glass photo-mask, a printed circuit board or a printing plate.
Optical scanners provide a well known solution for the above problems. One approach embraces a source of pulsed laser light for illuminating an acousto-optic Bragg cell functioning as a modulator. An electrical signal corresponding to optical data is fed to the acousto-optic Bragg cell for producing therein a varying diffraction grating for diffracting the laser light in accordance with the modulated electrical signal.
For example, in "Recent Components and Techniques in Optical Strobe Recording"(SPIE Proc. Vol 299, August 1981, pp. 98 to 103), R. A. Coppock and R. F. Croce disclose a wideband analog photorecorder comprising a transparent Bragg cell as an acousto-optical transducer and a pulsed laser positioned to sequentially illuminate a moving recording medium such as a photographic film through the cell. The acousto-optic cell is energized by an input electrical signal and the resulting sound wave passing through the cell diffracts the strobe light output beam from the laser so as to expose the film one line at a time with the optical analog of the signal.
Similarly, U.S. Pat. No. 3,851,951 (Eveleth) discloses a laser beam recording and playback system in which light is focused upon an image plane by interaction with frequency modulated acoustic pulses in a Bragg cell and is scanned across the image plane in accordance with the movement of the pulses along the cell.
In both of these prior art references, the Bragg cell functioning as an acousto-optic modulator produces optical scanning in one direction only of the image plane.
Typically, scanning in the direction perpendicular to the scan lines is achieved using a rotary polygon mirror. However, when high resolution imaging is required, polygon mirrors are generally unsatisfactory owing to their inherent manufacturing inaccuracies which give rise to such effects as pyramidal error and axis wobble. These effects give rise to the scanned line being not straight and, more significantly, being unpredictable. Optical corrections are to some extent available but render the resulting scanner very expensive.
In order to overcome the problems associated with the use of rotary polygon mirrors, it has been proposed to effect the desired scanning in the direction perpendicular to the scan lines by moving the image plane itself, such as is done, for example, in both of the above-mentioned prior art references. According to this approach, sequential lines of image data are scanned in the image plane, by transporting the image plane itself in a direction perpendicular to that of the scan lines.
In a practical laser plotter, for example, employing optical scanners of the type described, the resolution of the plotter depends on the number of pixels which can be separately imaged on each scan line and on the optical resolution between adjacent scan lines. Thus, in order to achieve high resolution, more pixels must be imaged on the image plane and, in order to achieve a given data throughput the resulting scanning time is decreased accordingly.
Various proposals have been made in order to increase the resolution along a single scan line by employing multi-spot modulators which, in effect, generate a multi-spot image of a data pattern permitting a plurality of bits to be recorded simultaneously. Such a system is shown, for example, in U.S. Pat. No. 4,577,932 (Gelbart) which discloses a multi-spot light modulator using a laser diode in which a single light pulse from the laser diode generates a multi-spot image of a data pattern, each spot corresponding to an active bit of the data pattern.
However, since prior art systems effect scanning in the direction perpendicular to the scan lines by mechanically transporting the image plane (e.g. photographic film), increasing the throughput in this direction also requires that the image plane be transported mechanically at a faster rate. In practice, it is generally dictated that high resolution laser scanners are used in conjunction with drum plotters rather than flat-bed plotters, since it is mechanically simpler to rotate a drum at high speed than to move a flat-bed plotter to and fro at high speed. However, it will be understood that regardless of whether drum or flat-bed plotters are employed, as soon as scanning normal to the scan line is determined by mechanical considerations, the optical scanning in the normal direction is necessarily limited with respect to that in the scan-line direction.
In addition, there is associated with the use of multi-spot and polygon scanners an optical distortion, since only a single beam can scan along a straight line. The peripheral spots suffer from a bowed scan line. This distortion is inherent to the .function.-.theta. lens employed in such scanners and can be cured by resorting to an .function.-sin .theta. lens. This, however, is at the price of increased complexity and expense and furthermore suffers from non-linearity in the scan direction.